Have Espruino tweet?

As @DrAzzy says, looks like that's probably not 'real' JavaScript, but is calling into internal V8 functions...

As there's no unicode it's simplified quite a bit - you could probably get away with:

function encodeURIComponent(s) { 
  var ok = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklm­nopqrstuvwxyz0123456789";
  var r = "";
  for (var i=0;i<s.length;i++) { 
    if (ok.indexOf(s[i])>=0) r+=s[i];
    else r+= "%"+(256+s.charCodeAt(i)).toString(16).s­ubstr(-2);
  }
  return r;
}